Video display systems based on spatial light modulators (SLMs) are increasingly being used as an alternative to display systems using cathode ray tubes (CRTs). SLM systems provide high resolution displays without the bulk and power consumption of CRT systems.
Digital micro-mirror devices (DMDs) are a type of SLM, and may be used for either direct-view or projection display applications A DMD has an array of micro-mechanical display elements, each having a tiny mirror that is individually addressable by an electronic signal. Depending on the state of its addressing signal, each mirror tilts so that it either does or does not reflect light to the image plane. The mirrors may be generally referred to as "display elements", which correspond to the pixels of the image that they generate. Generally, displaying pixel data is accomplished by loading memory cells connected to the display elements. After display element's memory cell is loaded, the display element is reset so that it tilts in the on or off position represented by the new data in the memory cell. The display elements can maintain their on or off state for controlled display times.
Other SLMs operate on similar principles, with an array of display elements that may emit or reflect light simultaneously, such that a complete image is generated by addressing display elements rather than by scanning a screen. Another example of an SLM is a liquid crystal display (LCD) having individually driven display elements.
To achieve intermediate levels of illumination, between white (on) and black (off), pulse-width modulation (PWM) techniques have been used. The basic PWM scheme involves first determining the rate at which images are to be presented to the viewer. This establishes a frame rate and a corresponding frame period. For example, in a standard television system, images are transmitted at 30 frames per second, and each frame lasts for approximately 33.3 milliseconds. Then, the intensity resolution for each pixel is established. In a simple example, and assuming n bits of resolution, the frame time is divided into 2.sup.n -1 equal time slices. For a 33.3 millisecond frame period and n-bit intensity values, the time slice is 33.3/(2.sup.n -1) milliseconds.
Having established these times, for each pixel of each frame, pixel intensities are quantized, such that black is 0 time slices, the intensity level represented by the LSB is 1 time slice, and maximum brightness is 2.sup.n -1 time slices. Each pixel's quantized intensity determines its on-time during a frame period. Thus, during a frame period, each pixel with a quantized value of more than 0 is on for the number of time slices that correspond to its intensity. The viewer's eye integrates the pixel brightness so that the image appears the same as if it were generated with analog levels of light.
For addressing SLMs, PWM calls for the data to be formatted into "bit-planes", each bit-plane corresponding to a bit weight of the intensity value. Thus, if each pixel's intensity is represented by an n-bit value, each frame of data has n bit-planes. Each bit-plane has a 0 or 1 value for each display element. In the simple PWM example described in the preceding paragraphs, during a frame, each bit-plane is separately loaded and the display elements are-addressed according to their associated bit-plane values. For example, the bit-plane representing the LSBs of each pixel is displayed for 1 time slice, whereas the bit-plane representing the MSBs is displayed for 2n/2 time slices. Because a time slice is only 33.3/(2.sup.n -1) milliseconds, the SLM must be capable of loading the LSB bit-plane within that time. The time for loading the LSB bit-plane is the "peak data rate".
As the pixel arrays of a spatial light modulator become larger and pixel resolution increases, the PWM method of providing greyscale places higher bandwidth demands on the delivery of data to the SLM. This is because the time within a frame allocated for the least significant bit becomes smaller. During this LSB display time, the pixel elements must be switched on and off very quickly and the data for the next bit must be delivered. Recent design efforts involving SLM-based displays have been directed to satisfying bandwidth requirements.
In addition to satisfying bandwidth requirements, an SLM-based display system should display its image with minimal artifacts. One potential artifact results from displays of objects in motion. The longer the time that a frame is illuminated, the more likely that a moving object will have a smeared appearance. This is a result of the fact that the viewer's retina and brain work together to integrate the display from frame to frame.